The present invention relates generally to an evaluating circuit for square wave signals.
It is known in the art that periodic analog signals are generated in incremental length or angle measuring systems by using a corresponding scanning unit in which a grid type incremental measuring graduation is scanned. In order to determine the direction of movement and to avoid errors due to the symmetry of the signals, such devices typically perform the scanning using four scanning fields which are offset by a quarter of the grid division in the measuring direction relative to each other.
Since in most cases the resolution corresponding to the grid division period of the measuring graduation is not sufficient, there have been a large number of proposals to subdivide the scanning signals.
For example, Swiss Patent CH-PS 407,569 discloses a subdividing circuit in which a large number of triggers are used to subdivide the scanning signals, thereby multiplying the scanning signals. The circuit disclosed in this reference however, provides no assurance that correspondingly fine subdivisions of the triggered signals will be synchronized with respect to their sequence and their flank spacing. Therefore, counting errors can occur in the incremental evaluation of the triggered square wave signals. Counting errors may depend on the degree of subdivision, on the measuring speed, on electrical and mechanical interfering influences (interference pulses, jarrings) and the like. These factors may lead to excessively small flank spacings, overlapping or inadmissible signal stages or signal sequences of the triggered signals.
Erroneous counts or measurements may not be perceptible from the display of the evaluating component during the measuring operation, since the last places of the display are illegible because of the rapid change of the display value. The measuring system must provide reliable measuring results which assure that all of the increments and all of the square wave signals recovered from the increments by the subdivision have actually been counted in the event that the measuring system comes to a standstill and if the measuring results are readable. Since the increments represent the measure for the path travelled over or covered, the sum, or in the negative measuring direction the difference, of all of the increments travelled over must correspond exactly to the display value.
All of the square wave signals may not be counted if, for example, the square wave signals which were recovered by subdividing the scanned signals have such a small flank spacing that they may not be reliably counted. In this event, the display provides a smaller measurement value than the value that has actually been covered by the measuring arrangement.
Similarly, the other faulty signal states discussed above may lead to erroneous measuring results.
JP Patent Abstract 59-105721 discloses a circuit for the synchronization of square wave signals. The disclosed arrangement uses a switching mechanism with bistable toggle stages. In the arrangement of this disclosure however, the synchronization is implemented by determining the nearest of several impulses.
U.S. Pat. No. 4,096,396 further discloses an oscillating system in which changes of the state of a plurality of timing networks depend on the states of the signals generated at the outputs of adjacent timing networks.
German Patent 35 23 551 discloses a process for synchronizing square wave signals using a circuit wherein a periodic auxiliary signal generator or clock times the sequence of the square wave signals. In the system disclosed in this reference changes of the signal state of a toggle stage are dependent on the signal state of adjacent toggle stages.
However, errors may still arise even with the use of this process. For example, in this arrangement the occurrence of two signal counting flanks catching up with one another may be perceive but not corrected.
Therefore in view of the above it is a primary object of the present invention to provide a more reliable evaluating circuit for square wave signals than the processes and circuits presently available in the art.